1. Field of the Invention
The present invention relates to a new organic borate compound, a nonaqueous electrolyte using the compound, and a lithium secondary battery and an electric appliance both using the electrolyte; the invention relating especially to a new organic borate compound high in oxidation resistance, a nonaqueous electrolyte improved in oxidation resistance by use of the compound, a lithium secondary battery and an electric appliance, both improved in cycle life by use of the electrolyte, and various applications of the electric appliance.
2. Description of the Prior Art
Lithium secondary batteries each made up of positive and negative electrodes capable of occluding and releasing lithium, a nonaqueous electrolyte, and other components, are high in energy density per weight and volume and in voltage. They are therefore hoped to be used as portable compact power supplies or as power supplies for electric automobiles. Since lithium secondary batteries have a high driving voltage of 3 V or more, they use a nonaqueous electrolyte wide in withstand voltage range. Compared with aqueous electrolytes, nonaqueous electrolytes have the drawbacks that they are low in electroconductivity and that a large portion of organic solvents suitable for an nonaqueous electrolyte are high in flammability (or low in flashing point).
For these reasons, researches for improving electroconductivity and, hence, the load characteristics of the battery, and researches for using a noncombustible or low-flammability organic solvent as the nonaqueous electrolyte for the battery, are taking place actively. An example of the former researches is reported in “A Collection of Preprints, the 40th Forum on Batteries, pp. 453˜454, (1999)”, and an example of the latter researches is reported in “A Collection of Preprints, the 40th Forum on Batteries, pp. 459˜460 (1999).” To improve the electroconductivity of a battery, it is necessary either to improve the dissociation characteristics of the supporting electrolyte to be used for the battery, or to select an organic solvent enabling the dissociation characteristics of the supporting electrolyte to be improved. For the electrolyte, organic lithium salt is proposed as lithium salt excellent in dissociation characteristics over the lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4) that is now mainly used. Above all, the lithium bis-(trifluoromethanesulfonyl)imide (LiN [SO2CF3]2) shown in Japanese Application Patent Laid-Open Publication No. Hei 05-326018 is known as a promising material having high solubility against a nonaqueous electrolytic solvent in comparison to an inorganic electrolyte. It is indicated, however, that the lithium bis-(trifluoromethanesulfonyl)imide (LiN [SO2CF3]2) has the drawback that it corrodes the aluminum used as the positive-electrode current collector for a secondary battery.
Also, organic lithium salt having boron to form its central ion and disalicylate to form its ligands, and organic lithium salt having boron to form its central ion and a benzenediolate derivative to form its ligands, are disclosed in Japanese Application Patent Laid-Open Publication No. Hei 07-65843. However, since these compounds have a benzene ring and are thus low in solubility, they cannot satisfy the electroconductivity or oxidation resistance required.
As disclosed in Japanese Application Patent Laid-Open Publication Nos. Hei 09-97627 and Hei 10-12272, to obtain noncombustibility or to ensure flameproofing, it is valid to use a fluorinated solvent. However, since the fluorine in fluorinated solvents is high in electron withdrawing ability, these solvents pose the problem that they deteriorate the electron releasing characteristics of their functional groups and, hence, the solubility and dissociation characteristics of lithium salt. Organic lithium salt has higher solubility than inorganic lithium salt, and is therefore a valid material.
For these reasons, the improvement of organic lithium salt is strongly desired as a promising material for improving the safety and performance of lithium secondary batteries.